1. Field of the Invention
This invention relates to a luminescent material and also to a light-emitting device.
2. Description of the Related Art
A light-emitting diode (LED) is generally constituted by a combination of an LED chip acting as an excitation light source with a luminescent material and emits various luminescent colors depending on the combination. In the case of a white LED device which emits white light, there is employed a combination of an LED chip which emits light having a wavelength ranging from 360 to 500 nm with a luminescent material. For example, there is employed a combination of an LED chip which is capable of emitting light in the ultraviolet or near-ultraviolet region with a mixture of luminescent materials. This mixture of luminescent materials may be composed of a blue luminescent material, a green or yellow luminescent material and a red luminescent material. It is required that the luminescent material to be used in a white LED device is not only capable of effectively absorbing the light ranging from the near-ultraviolet to blue regions, i.e., a wavelength of 360-500 nm which corresponds to the emission wavelength of the LED chip acting as an excitation light source but also capable of efficiently emitting visible light. However, these white LEDs are used in combination with a luminescent material exhibiting a wideband emission spectrum having a half band width of 80 nm or more. For this reason, the white LEDs are limited in color rendering and emission efficiency.
JP-A 2003-96446 (KOKAI) describes a Ce-activated yttrium silicon oxynitride luminescent material. This luminescent material is formed of a composition represented by Y2Si3O3N4:Ce.
Although they do not belong to yttrium silicon oxynitride, a Tb-activated La3Si8O4N11 luminescent material and a Tb-activated La3Si8−xAlxO4−xN11+x luminescent material are proposed in JP-A 2005-112922 (KOKAI). According to these luminescent materials, the practical excitation band of Tb3+ is limited to the range from the excitation spectrum shown in the drawing to an ultraviolet region in the vicinity of about 300 nm. For this reason, even if a light-emitting element which emits light ranging from near-ultraviolet to the blue region is employed as a light source, it would be almost impossible to obtain the emission of light.
Further, in the case of the ultraviolet LED, since the wavelength of light to be emitted therefrom is too short to use for illumination, the resin in which the luminescent material is dispersed would be prominently degraded. Moreover, the ultraviolet LED which emits light having a wavelength of about 300 nm is accompanied by problems that the manufacturing cost thereof is higher than that of the LED chip which emits light having a wavelength ranging from 370 to 440 nm and that the conversion efficiency of electricity into light is low.